This invention relates generally to beam index color television receivers and, more particularly, is directed to a circuit for controlling the color switching in a beam index color television receiver.
Beam index color television receivers are known wherein the display screen of the cathode ray tube has periodic index stripes in addition to the usual beam-excitable color elements, such as, red (R), green (G) and blue (B) phosphor stripes. The phosphor stripes, as is conventional, are arrayed in RGB triads, repetitively across the display screen so as to be scanned by the electron beam as the latter effects a horizontal scan in, for example, left-to-right traverse. As the electron beam scans the color phosphor stripes, it also scans the index stripes which, typically, also are phosphor stripes which emit light when excited by the scanning electron beam. In order to prevent light from the scanned index stripes from interfering with the displayed television picture, the index stripes are disposed on one surface of a thin metal layer which is thinly coated, as by vapor deposition, over the entire rear surface of the display screen, and the color phosphor stripes are disposed on the opposite surface of this thin metal layer, which layer is substantially transparent to the scanning electron beam but blocks the light which is emitted by the phosphor index stripes. A photo-detector responds to the light from each excited phosphor index stripe to produce a periodic signal whose frequency is equal to the frequency at which the phosphor index stripes are scanned. This periodic signal is then supplied to a band-pass filter so that, as the electron beam scans a horizontal line across the display screen, the photo-detector and band-pass filter generate a periodic index signal.
An example of a beam index color television receiver is disclosed in U.S. patent application Ser. No. 06/054,490 filed July 3, 1979, having a common assignee herewith.
The index signal which is derived from the scanning of the aforementioned phosphor index stripes is used to provide gating signals by which red, green and blue color control signals are gated onto, for example, the first grid of the cathode ray tube in repeated sequence. Since the index signal is derived from the scanning of the electron beam, the frequency of the index signal is related to the scanning velocity of that beam. Thus, the gating of the respective color control signals, referred to as color switching, desirably is synchronized with the beam velocity. This means that when the beam moves into scanning alignment with, for example, a red phosphor element, the red control signal is gated so as to modulate the beam with red signal information. Then, as the beam moves into proper scanning alignment with the green phosphor element, the red control signal is interrupted and the green control signal is gated so as to modulate the beam. Similarly, when the beam next moves into proper scanning alignment with a blue phosphor element, the green control signal is interrupted and the blue control signal is gated to modulate the beam. The foregoing gating sequence is repeated so that, as the beam scans the red, green and blue phosphor elements, it is concurrently and synchronously modulated with the red, green and blue color information.
In a beam index color television receiver of the above-described type, red, green and blue gates are provided for the red, green and blue color information signals, respectively, and each of these red, green and blue gates is opened individually and in sequence as the beam scans a horizontal line such that the respective color control signals are gated in time correspondence with the position of the beam at a color phosphor stripe that is associated with the gated color control signal. Typically, in such apparatus, for the purpose of obtaining color switching, that is, controlling the operations of the red, green and blue gates, the index signal from the band-pass filter is supplied to a phase-locked loop (PLL) circuit which is operative to provide a signal synchronized with the index signal and having a frequency twice that of the index signal. In the absence of time delays, as hereinafter referred to, the PLL circuit thus provides that, if the index signal undergoes a change in frequency due to, for example, a change in the scanning velocity of the electron beam caused by non-linearity of the horizontal deflection system, the red, green and blue gates nevertheless will be opened at the proper times, that is, at the times that the beam moves into proper scanning alignment with the red, green and blue phosphor elements, respectively.
However, the phase-locked loop, along with the photo-detector and band-pass filter, suffer from an inherent time delay. Thus, when there is a change in the scanning velocity there will be a small time delay until that change is reflected in the output of the phase-locked loop. This time delay can cause errors in the color switching synchronization, that is, the red, green and blue gates may open at slightly delayed times relative to the beam scanning of the respective color phosphor stripes. In other words, as a result of such time delay, when the electron beam is modulated by a particular one of the color control signals, the electron beam landing spot may be shifted from its desired position on the respective color phosphor stripe which is to be scanned. Since adjacent color phosphor stripes are separated by a black material formed of, for example, carbon or the like, the delay in phase of the color control signal may cause the electron beam landing spot to be shifted so as to overlap the adjacent black material or, in an extreme case, the next adjacent color phosphor stripe. This reduces the size of the landing spot on the respective color stripe with a resultant change in hue and a decrease in color saturation and relative luminance of the reproduced video image.
One apparatus which controls the electron beam in a beam index color television receiver so as to compensate for this time delay is described in the above-mentioned commonly assigned U.S. Patent Application. In this apparatus, a fixed delay or phase-shift circuit, which functions as a time adjustment circuit, is connected between the output of a voltage-controlled oscillator in the phase-locked loop circuit and the input of a phase comparator also in the phase-locked loop circuit. The voltage-controlled oscillator supplied an oscillating signal to the phase comparator through a frequency divider, so that a frequency-divided oscillating signal which is suitably delayed is fed back to the phase comparator for phase comparison with the index signal from the bandpass filter. In such apparatus, the time delay of the delay circuit is made to approximate the aforementioned time delay of the apparatus, so as to compensate therefor. However, the time delay of the apparatus may vary in accordance with changes in the scanning velocity of the electron beam and hence may not be accurately compensated for with a fixed delay circuit.